1. Technical Field
The present invention relates to a mobile communication system, and particularly relates to an apparatus and method for estimating power in a CDMA(code division multiple access) system which performs a power control of a down link or an up link in the CDMA mobile communication system and performs a power control about a CDMA signal by using an extended walsh code.
2. Description of the Prior Art
In a mobile communication system using a CDMA, such as a cellular mobile communication system or a personal communication service system, a plurality of mobile stations send/receive frames through a base station or a cell-site, in which the frame comprises symbols indicating a digitized voice or other data.
The transmission frames are affected by a fading through multipath. The fading effect relates to a transmission environment characteristic which reflects the transmission frames in accordance that a mobile station moves, and also relates to a coherence of other mobile station or other base station. At this time, the coherence is caused by a high transmitting power of a certain channel of a mobile station or a certain the base station.
In case of using a too low transmitting power, it may happen to lose the data because it is impossible to release the data of the transmitted frame. accordingly, a base station or a mobile station minimizes a fading or coherence through the multipath and controls a frame transmitting power for releasing the transmitted data safely at the same time.
By the reason as explained above, the system sends a power control command in order to regulate the transmitting power through respective separated channels by measuring the transmitting power from the frame which is transmitted between the base station and the mobile station.
The power control command is a transmitting power increment bit or a transmitting power decrement bit for maintaining an average value of a received power.
FIG. 1 shows a schematic diagram of a mobile communication system by using the CDMA in a prior art. Referring to FIG. 1, a PSTN(Public Switched Telephone Network) is a communication networks for a telephone subscriber provided by a communication network provider. It comprises a mobile switching center and a wire transmission line which performs a line exchange between the subscribers and a repeat call process in order that the mobile communication subscriber may communicate with other mobile communication subscriber or a wire network subscriber by using a mobile communication service.
Additionally, a base station controller 12 controls a power control by using an error rate of the received frame, and thereby generates a power control command in order that a mobile station 13 may send/receive the data by using a proper transmitting power through a base station A 14A or a base station B 14B.
Here, the power control command is a power control increment bit or a power control decrement bit having 1 to 2 bits of each frame.
FIG. 2 shows a schematic block diagram of an up link power controller of CDMA mobile communication system in a prior art.
Referring to FIG. 2, a CDMA signal of a spread spectrum sent from a mobile station, is received to a receiving antenna 20 of a base station through a CDMA channel and the received CDMA signal passes a down frequency process in a high frequency receiving unit 21.
The CDMA signal, which is processed as the down frequency in the high frequency receiving unit 21, is converted to a digital signal in an AID converter 22. A PN correlating unit 23 correlates the converted CDMA signal, by using a PN code, which is provided from a PN code generator(not shown in figure).
An output of the PN correlating unit 23 is provided to a decoder 24 for decoding and also provided to a power estimator 25 for estimating an average receiving power of a mobile station, in which the mobile station has sent the CDMA signal.
The decoder 24 decodes a code symbol about the received CDMA signal and provides a code error metrics to an external loop power controller 26, in which the code error metrics indicates a quality of the CDMA signal.
Accordingly, the external loop power controller 26 may recognize a sufficient transmitting power for sending the data by a mobile station(not shown in figure), and provides a power control reference value for using in the power control of an up link(a link from the mobile station to the base station), to a comparator 27.
The comparator 27 compares the average value of the mobile station receiving power which is estimated from the power estimator 25, and the power control reference value which is provided from the external loop power controller 26 and provides a deviation of the receiving power average value about the power control reference value to a power control bit generator 28.
The power control bit generator 28 provides a received power increment bit or a received power decrement bit according to the provided deviation, to a sending unit 29.
The sending unit 29 sends the received power increment bit or the received power decrement bit which is outputted from the power control bit generator 28, as well as a user data which is processed with an up frequency, to a mobile station through a sending antenna 30.
And then the mobile station regulates a receiving power according to the received power increment/decrement command and sends the user data at a transmitting speed in accordance with the regulated power.
The system controls a power control through the power estimator 25, the comparator 27, and the power control bit generator 28 as a closed loop power control. The closed loop power control is established after the base station measures a receiving power in accordance with a transmitting speed of the signal, which is received from the mobile station. And also the base station sends a proper power control command as well as the user data to the mobile station by comparing the received power with the power control reference value, in which the power reference value is predetermined in each base station. Thereby, the power control is performed by sending a signal as the regulated transmitting power in accordance with the power control command which is sent by the mobile station.
When performing the closed loop power control, the power control reference value is set according to several parameters, such as a moving speed of the mobile station, environment of the mobile station, and etc., in which the power control reference value is different from each mobile station. The base station controller properly regulates the power control reference value according to a receiving data frame error rate through the external loop power controller 40. The power control described above, is so called the outer loop power control.
The power control of the CDMA mobile communication in a prior art is usually applied to the up link. When performing a power control of a down link(a link from a base station to a terminal), it is required to measure a coherent rate about the CDMA signal which is received from the base station and as well as calculate a signal to a coherent rate after determining the transmitting speed of the currently received signal from the mobile station, because traffic channel of the down link has a variable transmitting power according to a transmitting speed after looking into the traffic channel which is similar to the power control. Therefore, it may happen that the delay of the transmission speed is larger when performing the power control. In other words, the delay of the transmitting speed can not follow a current variable CDMA channel environment
When the power control bit for the up link power control is sent through the traffic channel, the power control bit has a same power about all transmitting speed regardless of the sending speed. Therefore, it is possible to perform the power control by looking into the power control bit, which is transmitted through the previous traffic channel. However, an error may largely generated and thereby there are more possibility that the power control is to be wrong when calculating the coherent rate about the current received CDMA signal because the power control bit has a very short length.
In case of performing the power control by looking into a pilot channel besides the traffic channel, the previous pilot channel is used by the plurality of mobile stations which are involved in a signal base station. Accordingly, it is impossible to calculate a signal to coherent rate of the certain mobile station which performs the power control in the CDMA signal which is received from the plurality of mobile stations.
To solve the above problems it has been proposed to use the traffic channel and the pilot channel for controlling the receiving power.
All power control technology explained above depends on an ability of an estimator of estimating the receiving signal.
Here, the receiving signal comprises the receiving signal of a certain mobile station and a receiving signal of another mobile station, which uses an orthogonal code, which is different from each other. Accordingly, there is problem that an exact power control is difficult without dividing the receiving signal of other mobile station, which has coherence with a receiving signal of a particular mobile station, among the receiving signals of any channel.
Accordingly, in order to solve the problems in the prior art it is an object of the present invention to provide a method and apparatus for estimating power in CDMA system by using an extended orthogonal code which is extended according to a walsh function for an exact power control.
In order to achieve the above object in accordance with an embodiment of the present invention, there is provided a power estimating method in a CDMA system for distinguishing CDMA channels with a walsh code which is orthogonal to all CDMA channels, having the steps of making an extended walsh code having a new format, which is orthogonal to all CDMA channels, by extending a length(code stream) of the walsh code which is used for distinguishing the CDMA channels, according to a walsh function; and estimating a receiving power by correlating a receiving signal with the extended walsh code.
In addition the power estimating method can have a step of estimating a noise power in the receiving power by using a correlated value about the extended walsh code.
In order to achieve the above object in accordance with the present invention, there can be provided a power estimating apparatus in a CDMA system for distinguishing CDMA channels with a walsh code, including a extended walsh code correlating unit for extending a length (code stream) of the walsh code, used in distinguishing the CDMA channels, according to a walsh function in order to estimate a noise power included in a receiving signal, for making an extended walsh code having a new format, which is orthogonal to all CDMA channels, and for correlating the receiving signal with the extended walsh code, in which the receiving power is estimated with a correlating value of the extended walsh code.
The power estimating apparatus in a CDMA system can further include a traffic channel correlating unit of correlating a code symbol received through a traffic channel, and of outputting a correlating value in accordance with the correlated code symbol; a pilot channel correlating unit of correlating a code symbol received through a pilot channel, and outputting a correlating value in accordance with the correlated code symbol; and a power estimating unit of estimating an average value of the receiving power by using the correlating value of the extended walsh code correlating unit and the correlating values of the traffic channel correlating unit and the pilot channel correlating unit.
For obtaining the above object, in the power estimating apparatus and method in a CDMA system, the extended walsh code can be comprised by connecting a plurality of walsh codes successively.
In addition, the extended walsh code is comprised of walsh codes having different polarities from each other.
For accomplishing the above object, there can be provided the power estimating apparatus and method in a CDMA system, in which, when a vector of the extended walsh code is WM, a received signal vector rM(n) is
rM(n)=AnWM+ZM(n)
where An is a gain of a receiving channel, and ZM(n) is a noise vector at a moment of time nMT (T: walsh code length interval, M: number of the extended walsh code), and in which a correlating value yl(n) which correlates the receiving
yl(n)=MNAn+up(n)
signal with the extended walsh code, is
where MN is an overall length of the extended walsh code, and up(n) is a white noise vector, and in which a noise power which is estimated from above formulas, is             E      1        ⁡          (      n      )        =            E      ⁡              [                              "LeftBracketingBar"                                                            y                  1                                ⁡                                  (                  n                  )                                            MN                        "RightBracketingBar"                    2                ]              =                  σ        ^            2      
xe2x80x83Moreover, the present invention can provide the power estimating apparatus and method in a CDMA system, in which, when a walsh code vector which is allocated in the pilot channel is Wp, a received signal vector r(n) is
r(n)=SnAnWp+Z(n)
where An is a gain of a receiving channel, and Z(n) is a noise vector at a moment of time nT (T: walsh code length interval), in which a correlating value yp(n) which correlates the receiving signal with an allocated walsh code on the pilot
yp(n)=NAn+up(n)
channel, is
where N is a length of the walsh code, and up(n) is a white noise vector, and in which the overall receiving power which is estimated from the above formulas, is
and in which a power which estimated from the received signal, is
Ân2=Ep(n)xe2x88x92{circumflex over ("sgr")}n2